1. Field of the Invention
This invention relates generally to magnetoresistive (MR) sensors for reading data signals stored in magnetic media and specifically to a MR sensor with a special ferromagnetic transition layer for improved exchange-biasing of the ferromagnetic sensing layer.
2. Description of the Related Art
In magnetoresistive (MR) sensors, it is important to provide magnetic bias to suppress domain formation in the MR film element. A MR sensor detects magnetic field signals through the resistance changes in a thin film ferromagnetic MR strip arising from changes in external magnetic flux. Two magnetic bias fields are usually preferred for optimal MR element operation. A transverse bias field is usually provided to bias the MR strip so that it exhibits a linear response to external magnetic flux. This transverse bias field is normal to the plane of the magnetic recording medium and parallel to the surface of the MR strip. The second bias field is a longitudinal bias field that extends parallel to the surface of the magnetic medium and parallel to the lengthwise dimension of the MR strip. This longitudinal bias operates to suppress magnetic domain formation. Troublesome discontinuous changes in sensitivity and linearity occur in the outputs of thin-film MR sensors because the sensor dimensions are of the same order as domain dimensions. These discontinuities are known in the art as "Barkhausen noise" and are the result of sudden chaotic changes in domain wall positions with changes in applied magnetic field. The two simplest methods for avoiding Barkhausen noise in MR sensors are (a) to eliminate all domain walls or (b) to force such domain walls to be immobile.
A useful approach to eliminating domain walls in thin-film MR strips is to force the strip into a single domain by applying a permanent external magnetic bias. The related art is replete with useful methods for providing such external magnetic bias to thin-film MR strips. One approach to obtaining unidirectional anisotropy in the MR strip is through exchange interaction at the atomic boundary between an antiferromagnetic material and the ferromagnetic material making up the MR strip. Exchange anisotropy is well-known in the art as a form of surface anisotropy located at the phase boundary between a ferro- or ferrimagnet and an antiferromagnet. For instance, exchange anisotropy is known to occur on cobalt particles with an antiferromagnetic cobalt monoxide surface layer. The exchange coupling of the last plane of the magnetically-fixed antiferromagnetic lattice to the first ferro- or ferrimagnetic lattice plane leads to unidirectional vector anisotropy. This vector anisotropy behaves like a dc-bias field that displaces the hysteresis loop along the H axis and causes a finite anhysteretic magnetization in zero external field.
In U.S. Pat. No. 4,103,315, Robert D. Hempstead, et al. disclose a technique for minimizing domain walls in thin-film magnetic transducers that relies on the magnetic biasing effect of exchange anisotropy. Hempstead, et al. provide extensive detailed discussion of thin-film magnetic materials and fabrication methodology related to exchange bias applications and their patent is entirely incorporated herein by this reference.
Subsequent to the work by Hempstead, et al., many practitioners have proposed refinements to the exchange-biasing technique to incrementally improve MR sensor performance. In U.S. Pat. No. 4,663,685, Ching H. Tsang discloses a MR read transducer assembly in which the thin-film MR layer is longitudinally biased by exchange anisotropy only in the end regions. The bias field is developed by a thin film of antiferromagnetic material deposited in direct contact with the MR layer over the end regions. Limiting the longitudinal bias field to the end regions permits a central transverse bias field to maintain the central region of the MR layer in a linear response mode.
In U.S. Pat. No. 4,639,806, Toru Kira et al. disclose a thin-film magnetic sensor strip that is exchange-coupled to an adjacent permanently-magnetized ferromagnetic layer of higher coercivity to provide a longitudinal bias consisting of a combination of permanent magnetic field and exchange-bias field.
U.S. Pat. No. 4,713,708, issued to Mohamad T. Krounbi et al., discloses an exchange-biased MR sensor assembly that includes a third thin layer of soft magnetic material, where the antiferromagnetic exchange-biasing layer is removed in the middle region of the MR strip leaving only the thin film of soft magnetic material separated from the MR layer in the central region only by a decoupling layer that interrupts the exchange coupling so that traverse bias is produced only in the central region upon connection of a bias source to conductor leads, which are connected to the MR strip within the end region.
In U.S. Pat. No. 4,782,413, James K. Howard et al. disclose a MR sensor that uses an iron-manganese (FeMn) alloy as the antiferromagnetic exchange-biasing layer. The presence of the body-centered-cubic alpha iron-manganese alloy improves the longitudinal exchange bias in the ferromagnetic MR layer.
In U.S. Pat. No. 4,785,366, Mohamad T. Krounbi et al. disclose a MR read transducer that is exchange-biased over its entire length by a continuous thin film of antiferromagnetic material with a thin film of soft magnetic material disposed in the passive end regions such that the bias directions in different regions of the bias film are defined to produce optimum device performance. Krounbi et al. initialize the exchange-biasing layer to produce an effective bias field that is directed substantially longitudinally within the passive end regions and at some selected angle within the active central region of the MR sensor layer. Thus, the exchange-biasing layer is used to produce both the longitudinal and transverse bias fields.
In U.S. Pat. No. 4,809,109, James K. Howard et al. disclose an improved MR read transducer having an exchange-biased MR layer that is subjected to a thermal annealing process to create a ternary antiferromagnetic alloy at the junction between the ferromagnetic and antiferromagnetic layers. The ternary alloy provides the desired exchange-bias field at room temperature and exhibits an unusually high ordering temperature. Howard et al. neither consider nor suggest forming a variable-composition alloy layer in the ferromagnetic element to improve exchange-bias field levels, restricting their discussion to forming a new antiferromagnetic alloy at the interface between the two original films.
In U.S. Pat. No. 4,825,325, James K. Howard discloses a MR sensor that is longitudinally biased by the exchange anisotropy formed between the MR layer and a very thin layer of antiferromagnetic material, where the entire structure is covered with a protective film to prevent oxidation damage to the materials during subsequent thermal cycling.
In U.S. Pat. No. 4,967,298, Greg S. Mowry discloses an elongated MR sensor strip that is longitudinally biased to maintain a single domain sense region using exchange-biasing material atomically coupled to the strip at the ends outside of the central sense region in a manner similar to that of Krounbi et al above. Mowry's sensor strip is disposed between leading and trailing magnetic pole elements and the sensor strip is shielded from the trailing pole by a third shielding element.
In U.S. Pat. No. 5,014,147, Stuart S. P. Parkin et al. disclose an exchange-biased MR sensor strip employing an antiferromagnetic layer composed of iron and manganese alloyed in specified proportions. Parkin et al. specify the Fe.sub.(1-x) Mn.sub.x alloy, where x is within the interval of [0.3, 0.4].
Clearly, numerous practitioners in the art employ NiFe/FeMn exchange-biased films in MR sensor assemblies for domain suppression. In doing so, their exchange-bias field (H.sub.UA) is typically applied along the length of the MR sensor element. The magnitude of H.sub.UA must exceed a particular minimum to counteract demagnetization and coercivities in the MR material.
For instance, a NiFe/FeMn exchange-biased film with, say, 400 .ANG. (40 nm) of NiFe and 500 .ANG. of FeMn provides an exchange-bias field of about 25 Oersteds (2000 A/m). This 25 Oersted field is certainly adequate for most MR sensor geometries, especially for very long (i.e.: over 100 micron) designs. Unfortunately, this exchange-bias field magnitude varies sharply with changes in ambient temperature. For film thicknesses in the 400 .ANG./500 .ANG. range, H.sub.UA (T) varies from around 25 Oersteds at room temperature to zero at a critical temperature T.sub.cr of about 150.degree. C. Because this variation is substantially linear, H.sub.UA is reduced to only 12 Oersteds at the maximum device operating temperature of 90.degree. C. This 12 Oersted field value is generally only minimally sufficient to overcome the MR sensor coercivity of perhaps 10 Oersteds. There is accordingly a clearly-felt need for exchange-biased film structures that provide a higher exchange-bias field value at H.sub.UA at maximum device operating temperature.
One approach for increasing the exchange-bias field H.sub.UA at 90.degree. C. is to introduce film changes to somehow increase H.sub.UA at room temperature (20.degree. C.) without incurring any significant reduction in critical temperature T.sub.cr.
The decline in H.sub.UA at higher operating temperatures is even more of a problem when using one of the corrosion-resistant FeMnX alloys (where X includes one of a group of elements including Cr, Rh and Ti) as the antiferromagnetic layer because H.sub.UA is significantly lower at room temperature with these alloys. Thus, there is a clearly-felt need for a technique that enhances H.sub.UA in corrosion-resistant exchange-biased MR assemblies.
These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.